Disturbance Ecology

The role of disturbance in molding the structure and function of biological systems is a major paradigm in ecology. Indeed, as the human population continues to grow, with attendant expansion of agricultural and industrial enterprises, the future of ecology may reside in understanding the ways in which disturbance modifies the structure of communities, as well as the way component species affect energy flow and nutrient cycling of disturbed ecosystems (Willig and Walker, 1999; Walker and Willig, 1999). Much of my research in the Luquillo Mountains LTER Program in Puerto Rico focuses on the response of the biota to various elements of the disturbance regime. Along with my students, I have documented the way in which both populations and communities of vertebrates and invertebrates respond to infrequent, but large-scale and high-intensity disturbances such as hurricanes (Gannon and Willig, 1994; Gannon and Willig, 1999; Willig and Gannon, 1996; Willig et al., 1999; Garrison and Willig, 1996; Secrest et al., 1996), as well as to frequent, but small-scale and low-intensity disturbances such as tree falls (Alvarez and Willig, 1993). Nonetheless, the legacy of human disturbance may remain for long-periods of time (scores of years), and modify the impact of subsequent natural disturbances (Secrest et al., 1996; Willig et al., 1996). I recently developed a conceptual model of disturbance that includes subsequent trajectories of recovery by the biota (Willig and Walker, 1999). Efforts of synthesis regarding disturbance, are reflected in three products. The first is a Special Feature of Biotropica that I co-edited with three other LTER scientists (Zimmerman et al., 1996). The second is a series of papers in The Ecology of Disturbed Ground (part of the Elsevier Ecosystem Series) on disturbance in general (Walker and Willig, 1999; Willig and Walker, 1999), as well as on the response of animals to disturbance and their role as agents of disturbance (Willig and McGinley, 1999). The third is a synthesis volume for the Luquillo Mountains LTER site which I am co-editing with other scientists from Puerto Rico. Two ongoing biodiversity projects in South America, one in Paraguay and one in Brazil, investigate the effects of disturbance on composition and diversity of tropical habitats. Research in the Tapajos National Forest seeks to assess the differential responses of birds and bats to natural and anthropogenic disturbance in Amazonian Terra Firme forest.

 

In general, we are quantifying changes in the abundance of species and the composition and diversity of communities between primary forestand forested areas that have been subjected to low impact logging. In particular we are focusing on gap and non-gap regions of primary forest, and small gaps (removal of single trees), large gaps (patios created for storage of extracted timber) and undisturbed regions of managed tracts of forest. Research in Paraguay has documented the composition and diversity of the small mammal fauna at 25 sites distributed throughout the seven major biomes of the country (Willig et al.,2000). In addition, my students (R.D. Stevens and P.M. Gorresen) examined the response of populations and communities of bats - critical agents of seed dispersal and flower pollination - to forest fragmentation in the Interior Atlantic Rain Forest of eastern Paraguay.

 

Canopy Trimming Experiment

Four treatments of the CTE.

Tropical forests in the Greater Caribbean Basin are hurricane mediated, as much of the spatiotemporal variation in the structure, physiognomy, and composition of these ecosystems arises as a consequence of these intense storms. The ecological impact of these relatively infrequent but intense and large-scale disturbances has been well documented for many taxonomic groups, including plants, microbes, gastropods, insects, and bats. However, the mechanistic bases of observed responses to hurricane-induced disturbance are poorly understood. Severe hurricanes create gaps in the forest canopy, relocating the leaves and branches to the soil surface. Several associated physical changes occur as a consequence of this canopy disturbance: increased light levels at or near the soil surface, a corresponding increases in temperature and evaporation from litter and soils. In addition, the deposition of debris from the canopy represents a significant alteration to the structure of the understory as well as a major change in the quantity and quality of carbon-inputs into detrital food webs.  Observational studies cannot disentangle the various mechanistic bases of changes to the environment that arise from hurricane disturbance. At the Luquillo Experimental Forest, a NSF Long Term Ecological Research site, a canopy trimming experiment (CTE) was designed to isolate the effects of canopy loss and debris deposition on plants, animals, and ecosystem processes. Large blocks of forest were experimentally manipulated using a replicated factorial design, with treatments representing: undisturbed forest, debris deposition only, canopy loss only, and both canopy loss and debris deposition. This allowed for the simultaneous evaluation of the responses of many taxonomic groups and ecosystem processes, including phosphorous movment in leaf litter, soil chemistry, nutrient dynamics, herbivory, forest floor microbial communities, fungi, ferns, trees, terrestrial gastropods, canopy arthropods, and frogs (upcoming special issue in Forest Ecology and Management).

 

Related Manuscripts

Schowalter, T., M.R Willig, and S.J. Presley. 2014. Canopy Arthropod Responses to Experimental Canopy Opening and Debris Deposition in a Tropical Rainforest Subject to Hurricanes. Forest Ecology and Management http://dx.doi.org/10.1016/j.foreco.2013.12.008.

Willig, M.R., C.P. Bloch, and S.J. Presley. Experimental decoupling of canopy opening and debris addition on tropical gastropod populations and communities. Forest Ecology and Management (Accepted).